1. Field of the Invention
This invention relates to thin panel television displays. More particularly, this application describes a thin panel display technology based on optical waveguides. Specifically, a technique for tapping light out from the cores of closely spaced parallel optical waveguides is described which enables images to be formed on large viewing screens.
2. The Need for a Flat Panel Display
Since the cathode-ray tube (CRT) television was first commercially introduced in the 1930's, display manufacturers have attempted to make a screen which can be hung on a wall like a painting. This "moving painting" should ideally be flat, bright, large, inexpensive, rugged and with a high enough resolution, at least 1024.times.1024 color picture elements or "pixels", to produce a high definition television (HDTV) image. The ability to make a thin picture screen will have a great impact on the television viewing experience. For example, thin panel screens will facilitate in-home entertainment centers. A home viewer will be able to watch theater-quality movies without bulky and expensive projection equipment.
3. Prior Art
Liquid crystal, gas plasma, vacuum fluorescent and electroluminescent technologies are presently considered the most promising for use in thin panel displays. These technologies are similar in that they all rely on a matrix of electrodes. Electrical signals applied to the matrix of electrodes control a working medium. Commonly used working mediums include liquid crystals, neon, and phosphors. The working medium is typically sandwiched between the matrix of electrodes. The light emitted from a screen pixel is regulated by energizing its associated electrodes. If the proper electrical signals are rapidly applied to the electrodes, still and moving images can be formed.
In practice, large displays based on matrices of electrodes have yet to be constructed. Increasing the screen size makes the electrode geometry and composition more difficult to control during the manufacturing process. As a result, large matrix-based electrode panels tend to have irregular screen brightness regions and defective picture elements. While screen brightness irregularities can be compensated for by electronically processing picture signals before they are applied to the display, defective picture elements can render a panel useless. Defective picture elements in very large displays currently make large matrix-electrode panels prohibitively expensive to produce. It is unlikely wall-sized matrix-electrode displays will become economically feasible in the near future.
In an effort to address these problems, a variety of less well-known display designs have been proposed. One particular design approach is based on the use of optical waveguides. Optical waveguides are capable of carrying high-intensity light long distances, with little attenuation. These efficient light carrying properties make waveguides well suited for use in large screen displays.
One prior art waveguide design employs optical fibers to magnify images from an image source. For example, if the image source is a CRT, and a plurality of optical waveguides are butted to the face of the CRT, the individual fibers associated with each pixel on the CRT can carry light to a separate, larger, display panel. Each pixel from the CRT is mapped, one-to-one, onto the large panel by a dedicated fiber. If the panel is much larger than the original CRT, the fibers collectively act to magnify the picture image. Extremely large screens can be built using this fiber optic magnification technique.
When applying fiber optic magnification to HDTV, a number of problems arise. A fiber optic magnification system at a HDTV resolution requires millions of separate optical fibers. Producing a screen with millions of optical fibers is both complicated and expensive. Consequently, fiber optic magnification techniques are generally considered impractical for use in high resolution displays.
An improved optical waveguide technique is based on removing, or tapping out, light from a waveguide core before it reaches the end of the fiber. Instead of using one fiber per pixel, as described above, a single fiber can be made to show many pixels. For example, multiple taps can be placed along the length of a single fiber. If the taps are scanned in rapid succession, a single fiber will appear to possess many separate light emitting elements. Tapping light out, and allowing it to be seen by a viewer, greatly reduces the number of waveguides needed in a display.
A number of investigators have pursued this multiplexed, waveguide/tap, approach. The few resulting waveguide/tap designs are similar in that they have all attempted to find an efficient means of tapping light out from the waveguide core. The light tapping techniques discussed in the prior art incorporate electro-optic, thermo-optic and liquid crystal tap elements.
Perhaps the most well developed waveguide/tap display technology relies on thermally-induced phase changes in a liquid core waveguide. In this approach, a liquid core fiber is heated externally through the cladding. At a critical temperature the liquid core vaporizes, causing a sudden drop in the core's refractive index. Light traveling through the heated, vaporized, core region is caused to scatter and can be seen by a viewer.
While the liquid core technique is promising, it has several major drawbacks which are representative of the flaws in other existing waveguide/tap designs. First, the time required to cycle through a liquid-vapor-liquid phase transition is typically in the millisecond range. A millisecond is far too long to enable a single fiber to display and update thousands of pixels/second.
Fiber taps must update pixels at rates greater than 30,000 pixels per second to produce a high resolution waveguide/tap display.
Second, the manufacturing processes needed to make a system of parallel liquid waveguides are new and have not been well developed. Since the manufacturing processes are new, liquid waveguides are presently expensive to produce. These two problems, designing high speed taps and economical optical waveguides, are common limitations to all previously proposed waveguide/tap displays. In other words, waveguide/tap displays have been described in theory, but have not yet been possible to build.
Consequently, while several waveguide/tap display methods have already been proposed, non can be used to produce an economical large panel display. Either the taps have been too slow or the fiber elements have been too costly to produce. Therefore, while preliminary work has shown optical waveguide/tap displays to be promising, improvements in the light tapping, waveguide forming, and source illumination elements must be made before this technique can be commonly applied to large panel screens.
Examples of relates patents included U.S. Pat. No. 3,871,747 issued Mar. 18, 1975 to Ronald Andrews; U.S. Pat. No. 4,640,592 issued Feb. 3, 1987 to Nishimura et al.; U.S. Pat. No. 3,856,378 issued Dec. 24, 1974 to Brandt et al.; U.S. Pat. No. 3,619,796 issued Nov. 9, 1971 to Seidel et al; U.S. Pat. No. 3,655,261 issued Apr. 11, 1972 to Change.
Other related art is disclosed by Manhar L. Sha, "Fast acousto-optical waveguide modulators", Applied Physics Letters, Vol 23, No. 2, Jul. 15, 1973, pp. 75-77; A. I. Gudzenko et al., "Acoustooptical modulator using coupled plane waveguides", Opt. Spectrosc., (USSR) 47 (4), October, 1979, pp. 427-428; G. B. Brandt et al., "Bulk acoustic wave interaction with guided optical waves", Applied Physics Letters, Vol. 23, No. 2, Jul. 15, 1973, pp. 53-54; B. L. Heffner et. al., "Switchable fiber-optic tap using acoustic transducers deposited upon the fiber surface", Optics Letters, Vol. 12, No. 3, March 1987 pp. 208-210.; Ralph Th. Kersten, "Integrated optical acousto-optic switching", Spie vol. 517 Integrated Optical Circuit Engineering, 1984, pp. 258-266; L. Falcou et. al., "Switching characteristics of a piezoelectrical actuated evanescent-wave directional coupler", Electron. Lett., Vol 23, 1987, pp. 469-470; K. Lieu "Single-mode-fibre evanescent polarizer/amplitude modulator using liquid crystals", Opt. Lett. Vol 11, 1986, pp. 180-182; Manhar L. Shah, "Fast acoustic diffraction-type optical waveguide modulator", Applied Physics Letters, Vol. 23, No. 10, Nov. 15, 1973; T. Tamir et. al., Integrated Optics, Topics in Applied Physics, Vol 7, Springer-Verlag 1985.